Transmitter Optical Sub-Assemblies (TOSAs) are well known in the art of optical networks. A TOSA operates as an electro-optical converter for use in data communications and telecommunications applications. It transforms electrical signals into corresponding optical signals that are then focused into an optical fiber. Once the optical signal reaches its destination, it is typically focused into a ROSA (Receiver Optical Sub-Assembly) for conversion back into a corresponding electrical signal.
A TOSA typically includes a diode laser for producing an optical signal and a lens for focusing the optical signal into the input end of an optical fiber. Diode lasers (e.g. distributed feed-back diode lasers) are typically sensitive to back reflected light (e.g. light reflected off of the input face of the optical fiber back into the diode laser). Therefore, TOSAs also typically include an optical isolator located between the laser and the optical fiber that allows light to pass from the diode laser to the optical fiber while preventing any back-reflected light from reaching the diode laser.
A common optical isolator is a Faraday rotator device having an input linear polarizer, a garnet crystal and an output linear polarizer. Normally, the transmission axis of the input linear polarizer is aligned to the linear polarization of the diode laser output to maximize light transmission through the input linear polarizer. The garnet crystal is subjected to a saturating magnetic field, making it a Faraday rotator having a thickness chosen such that the polarization of transmitted light is rotated by approximately 45 degrees. The polarization is rotated in the same direction regardless of the propagation direction of the light. The transmission axis of the output linear polarizer is oriented at approximately 45 degrees relative to that of the input linear polarizer to maximize the transmission of the light from the diode laser that has passed through the input polarizer and Faraday rotator.
Any light that is reflected back toward the diode laser is first incident upon the output polarizer, which passes only light linearly polarized along its axis. The polarization of this admitted light is rotated by approximately 45 degrees by the garnet crystal, and ends up being orthogonal to the transmission axis of the input polarizer. At the input polarizer, this light that is polarized orthogonal to the transmission axis is either absorbed or reflected away from the fiber. Optical isolators are used extensively and have excellent performance with typical insertion loss of <0.3 dB and isolation of>25 dB (reduction factor for reflected light).
TOSAs must satisfy certain bandwidth and power requirements in order to function properly for use in practical networking applications. These characteristics both vary as a function of the electrical current through the diode laser. Therefore, many times it is not possible to satisfy both the power requirement as well as the bandwidth requirement simply by adjusting the diode laser current. One solution is to set the diode laser current to meet the bandwidth requirement even though it may possibly exceed the power requirement, and then attenuate the diode laser output as needed to then meet the power requirement. A prior method of achieving this attenuation is to insert an optical plate with an attenuating thin-film coating, or to use a neutral density filter to attenuate the optical power. Such an optical filter has been used with an optical isolator (see for example U.S. Pat. No. 6,297,901). Adding additional optical element(s), however, is disadvantageous because of the cost of the optical element(s), the creation of additional optical interfaces, and the increased possibility of a failure related to the added optical element(s) (e.g. coating failure, mechanical mounting failure, alignment failure, surface contamination, etc.). Additionally, the attenuation of the added passive optical element(s) is not adjustable, thus requiring a series of optical elements to be created and stocked.
A variety of active devices have also been used to achieve variable attenuation. For example, it is possible to vary the magnetic field applied to the Faraday rotator to adjust the rotation angle of the light relative to the output polarizer (see for example U.S. Pat. No. 6,384,957). However, this solution increases the complexity and cost of the TOSA, and decreases the effectiveness of the optical isolator in completely blocking back-reflected light.
There is a need for a simple, inexpensive way to achieve arbitrary and continuously adjustable attenuation values for laser light focused into an optical fiber, without adding to the cost or complexity of the TOSA.